test: fix test-https-agent-additional-options.js

`value` was always being assigned to an `undefined` property of an
Array. Thus, the assertions that depended on `value` being defined were
never being checked. Assign `value` the correct...er...value.

PR-URL: #38202
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

test/parallel/test-https-agent-additional-options.js

@@ -61,8 +61,8 @@ function variations(iter, port, cb) {
         server.close();
       } else {
         // Save `value` for check the next time.
-        value = next.value.val;
         const [key, val] = next.value;
+        value = val;
         https.get({ ...getBaseOptions(port), [key]: val },
                   variations(iter, port, cb));
       }